Heritability is a fraught topic. It comes up repeatedly on this weblog, but even long time readers can be confused as to its implications, as evidenced by the incorrect inferences made from their own understanding of the concept. The most common problem is that too often people think that heritability is just a scienced up version of the colloquial idea of some traits being “more genetic” or “less genetic.” It’s not that at all. Traits totally specified in their details by genetic pathways can be non-heritable. That’s because heritability looks at the association between parents and offspring on a trait and attempts to separate the population-wide proportion of the variation attributable to genes and not attributable to genes. When you have a genetically specified trait, like the number of human fingers, you have no real variation within the population to work with (with some rare exceptions). It doesn’t make sense to talk about the heritability of the number of fingers, because this is a fixed trait in the human species.

In contrast, height is a perfect trait to illustrate heritability. Unlike behavioral or cognitive traits its measurement is clear, distinct, and uncontroversial. Additionally, there’s a normal distribution of the trait. By that, I mean that there is a bell curve from tall to short, with a median at the peak of the distribution. Not only does height vary within the population, but it varies across populations, and it varies within families. When considering the heritability of height there’s a lot to grapple with and ponder for a relatively easy to measure characteristic.

Intuition will tell you that parents and offspring tend to correlate positively in terms of height, but the trait is imperfectly correlated. Comparisons between identical and fraternal twins can allow us to partition the rough effects of genes vs. non-genes on the trail value. Identical twins share ~100% of their genes, while fraternal twins share ~50% of their genes, so the correlation in trait value should diminish in proportion to the decreased genetic similarity and the effect of genes on the trait value. In plain language, if genes don’t account for any of the variation in the trait value then identical and fraternal twins should exhibit the same correlations on a trait. In contrast, if genes control almost all the variation on the trait value then identical twins should be rather close in value, while fraternal twins far less, because the twins in the second case differ a great deal genetically while they do not in the former case.

But even with a trait like height environment matters, more or less. Again, intuition tells you that if you starve a child during a critical period they may never attain their height as might be inferred from their genetic potential. Therefore heritability is informative in light of background conditions. In the developed world the heritability of height is ~0.80 to ~0.90, which means that 80-90% of the variation in height can be explained by variation in genes. This is not so in other regions of the world, where environmental variables such as nutritional stress loom larger. There is no fixed proportion by which height is “more genetic” or “more environmental.”

There’s much more that could be said. But much of it has been said, and by those better able to say more. So I’ll move to my value-add: what is the profile of those who know and don’t know what heritability is? In the survey I asked: “Do you know that narrow-sense heritability is the proportion of the phenotypic variance due to the additive genetic variance?” The terminology may seem opaque to some of you, but to those who knew what heritability was it wouldn’t be. ~33% of you out of ~550 (the survey is still live) recognized what narrow-sense heritability was by the definition I offered. I was frankly nicely surprised, since I know many of you skim for topics of particular interest to you rather than “deep dive” into quantitative genetics, which I don’t talk much about in depth in any case.

The results from the survey will be analyzed in three parts:

– In the first knowledge of heritability is a function of another variable. In other words below you a see table which shows that 34 percent of males recognized the definition of narrow-sense heritability and 29 percent of females did. The average is ~33 percent because 80 percent of respondents were male vs. 20 percent female. Remember that the proportion in the aggregate pool is ~33 percent.

– The second section focuses on the differences between people who know what narrow-sense heritability is, and those who do not. So in this case you have other factors as a function of heritability as a categorical variable.

– A final section examines some scatter plots of estimated height vs. IQ heritabilities. An interesting point to remember here is that many people who weren’t totally clear on what narrow-sense heritability was nevertheless offered up a heritability estimate, probably because they got the gist of what the value was.

Knows what “narrow-sense heritability” is

Male

34

Female

29

Some university

16

Bachelors

30

Masters

30

Advanced degree

53

Supports abortion on demand

32

Rejects abortion on demand

33

Accepts supernatural

25

Rejects supernatural

34

Mathematics background

34

Computer science background

30

Physical sciences background

27

Life sciences background

60

Psychology background

38

Economics background

37

Anthropology background

48

Other social sciences background

19

Philosophy background

36

Other humanities background

23

Law background

45

Medicine background

50

Has engaged in recreational genomics

47

Has not engaged in recreational genomics

28

Can program

34

Can not program

32

Is interested in transhumanism

29

Is not interested in transhumanism

34

Need to always be on guard against genetic determinism

35

Genetic determinism is a concern, but often overblown

34

We don’t have to be concerned about genetic determinism

34

Biological sex differences in behavior trivial

6

Biological sex differences in behavior very modest

29

Biological sex differences in behavior somewhat significant

32

Biological sex differences in behavior very significant

42

Biological race differences in behavior trivial

25

Biological race differences in behavior very modest

32

Biological race differences in behavior somewhat significant

41

Biological race differences in behavior very significant

40

Biological within population differences in behavior trivial

18

Biological within population differences in behavior very modest

27

Biological within population differences in behavior somewhat significant

39

Biological within population differences in behavior very significant

46

Biological across sibling differences in behavior trivial

21

Biological across sibling population differences in behavior very modest

30

Biological across sibling population differences in behavior somewhat significant

38

Biological across sibling population differences in behavior very significant

40

Has made a non-trivial edit to Wikipedia

38

Has not made a non-trivial edit to Wikipedia

30

In the plots below red = those who know what narrow-sense heritability is, and blue = those who do not.

Now let’s look at the scatter plot of height and IQ heritabilities, with shading proportional to the N within a bin.

The correlation between the two estimates is ~0.52, ~0.60 for those who know what narrow-sense heritability is, and ~0.47 for those who don’t. It’s clear that a lot of people who didn’t know the definition for heritability just guessed 0.50 for both traits (reasonable I think). Just to be clear, in developed nations the heritability for height is pretty well known to be in the ~0.80-0.90 range. That explains the peak there, especially for those who know what narrow-sense heritability is. For IQ there are a large range of numbers, from ~0.30 up to ~0.80.

Thanks for exposition. As one who has mental blocks in this area let me ramble a little. Let us take your height example. Suppose, let us say in a population of cattle, to make this only slightly less gory, before they are old enough to breed they pass under a killing fence. If they are tall enough they are culled and become veal. Let us further say average height of the herd will become smaller and if that herds genetics are compared to another there will be a definite variation. This whole thing really shouldn’t affect the amount of heritability of height in the cattle correct?

Let us say we do this for thousands of years such that the several of the genetic variations that cause tall cattle are lost from the population totally and nobody noticed that. According to the definition wouldn’t there now be less heritability of height measured in cattle by the folks who didn’t know there were currently non-existing possibilities?

I contrived this funny scenario because I feel like on the one hand the “heritability” factor could be partly caused by the environment, or rather it was caused by the environment in the past by the pressure that created the current genetic profile of a population, and it (heritability) only looks at the current state, not possible other genetic profiles that are not in the population under study but that might be “available” under correct conditions.

Does this make sense? These points make the heritability only really useful as a pointer for further looks. I think I get the base mechanics of the model: height is partially due to genetics, such that even if constraints like amount of food are removed, different genetic profiles are correlated with height in ways that have decent predictive value. But I guess I feel like there are always non-genetic constraints (or unconstraints, like the assumption of good nutrition) that can affect a trait. Thus the heritability correlations are just a little part of a bigger model to me, and I am not sure that they are the right way to split things out.

I this this is just my understanding issue.

http://sep.stanford.edu/sep/jon/ Jon Claerbout

Next time you give the quiz, consider “Engineering” as a possible background.

Ian

According to the definition wouldn’t there now be less heritability of height measured in cattle by the folks who didn’t know there were currently non-existing possibilities?

Probably not. If height was controlled by a single gene, then yes, in a scenario like this, you might lose the ‘tall’ allele. But height is additive. There will still be variability in this new population, it will just be variation around a different mean.

toto

Razib: what about putting a quick, crisp definition of heritability at the beginning of every post that discusses it? Just something like, “Heritability is the proportion of the variance in a trait, within a given population, that is caused by genetic differences” – preferably in bold. That might help spread the knowledge.

#1: Basically, you’re right. Heritability is population-dependent. Two different populations, living in different conditions, may have different values of heritability for the same trait.

That’s one of the reasons why height is so useful as an example, as Razib said. Everybody understands that malnutrition will dramatically affect your height, and yet height is 90% heritable in western countries – simply because, as the environmental variation is low, mechanically the genetic variation has a higher impact on total trait variation.

Notice that you can already infer this from the simple definition above.

http://sinophibe.blogspot.com/ Klortho

I’m just a little bit skeptical of the self-reporting of the knowledge of heritability, and how that might bias these results.

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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com